Diesel engine combustion is characterized by the injection of a self-ignitable fuel under very high pressure through an injector nozzle into a combustion bowl arranged in the piston. After the atomization and vaporization of the fuel in the combustion air heated to high temperatures through compression, a mixing of the already vaporized fuel components with the combustion air takes place. This carburetion is achieved on the one hand through the distribution of the fuel by means of 6- to 8-hole injector nozzles, and on the other hand through swirling air generated in the inlet channels. The chemical processing of the fuel-air mixture then occurs through cracking of the relatively long fuel molecules and the formation of active radicals. If the concentration of active radicals is sufficiently high, the self-ignition of the fuel-air mixture begins in the form of a chain reaction. The time required for the physical and chemical carburetion processes is referred to as the ignition delay. Due to the short amount of time available for carburetion as a result of the direct injection of the fuel, the remaining combustion takes place in a fuel-air mixture with locally non-homogenous fuel distribution. The ignition phase of this “non-homogenous” fuel-air mixture is characterized by the occurrence of “ignition cores” in already ignitable regions of the mixture. As combustion continues, the fuel droplets react first that have reached the compressed hot combustion air at the beginning of the injection of fuel and for which a relatively long time is thus available for mixture processing. Due to the resulting relatively faster combustion reactions, this combustion phase, which is also referred to as the combustion of “pre-mixed” mixture, is characterized by higher combustion temperatures and thus greater thermal nitrogen oxide formation and less soot formation. However, during this first combustion phase, only a certain portion of the fuel-air mixture determined by the length of the ignition delay is combusted. The remaining mixture quantity that is not yet ignitable at the moment of the start of combustion and depends on the required engine load, is first processed as the combustion process continues through steeply rising gas temperatures and an intensive charge motion and then combusts in part under lack of air at a relatively low rate of combustion. This combustion phase, which is also called “diffusion-controlled” combustion, is characterized by initial soot formation occurring as a result of local air deficiency and subsequent incomplete post-oxidation of soot with simultaneously reduced nitrogen oxide formation.
The various phases of the combustion process and the corresponding mechanisms of pollutant formation yield a relationship between the nitrogen oxide and soot emissions that is typical of diesel engine combustion and is also referred to as NOx particle trade-off. This NOx particle interrelation means that, if the engine operation parameters (e.g., the injection timing) are adjusted for low NOx emissions, a simultaneous rise in soot/particle emissions is unavoidable.
In order to comply with the legally prescribed pollutant emissions of diesel engines, measures are taken both within the engine and externally in the form of exhaust gas treatment processes. The most important of the measures taken within the engine to improve the fuel-air mixture formation consists in the use of high-pressure injection systems, which enable injection pressures of greater than 200 MPa. The improved breakdown of the injection spray into smaller fuel droplets achieved in this way leads to improved mixing of the fuel with the combustion air and thus to fewer rich mixture zones and, accordingly, to substantially lower soot and particle emissions. Due to the higher combustion temperatures, the carburetion intensified in this way results in higher NOx emissions, which are to be avoided through increased excess air by means of increased charge pressures and optimized injection quantity curves. Another measure is exhaust gas recirculation (EGR), which is increasingly also being used in heavy commercial vehicle engines. However, the exhaust gas return rate and thus also the possible NOx reduction through decreasing oxygen content in the combustion air are limited once again by rising soot and particle emissions.
Since the described measures taken within the engine to reduce pollutants are insufficient for bringing emissions to below the exhaust gas limits, DeNOX catalytic converser systems with urea as a reduction agent and, separately from diesel engines, particle filter systems that are known for use in passenger vehicles are being used in newly approved commercial vehicle engines that must meet the requirements of Euro 5 and Euro 6 pollutant categories. To achieve the emissions goals, the raw emission behavior of the diesel engine must be adapted to the exhaust gas treatment systems used. For instance, in typical Euro 5 diesel engines, the particle emissions are reduced to the corresponding limits through flexibly tunable Common Rail (CR) injection systems with 160 to 180 MPa injection pressure, whereas the nitrogen oxide emissions are usually reduced sufficiently through the use of a urea-based DeNOx system. Sometimes, the NOx emissions are also reduced through a combination of exhaust gas recirculation and downstream DeNOx system. Which economically tenable combination of methods is used depends essentially on the raw emissions of a diesel engine. In diesel engines of the Euro 6 emissions level, the use of additional optimized injection systems with injection pressures from 200 MPa and up are required in order to further reduce particle emissions, as well as more efficient EGR-DeNOx system combinations. In particular, the use of EGR systems with substantially higher EGR rates and of DeNOx systems with NOx conversion rates of up to 90 percent is necessary. If the NOx emission limits cannot be complied with through the described use of EGR and DENOx systems, the additional use of a particle filter system with commensurately adapted tuning of the EGR and DeNOx system is unavoidable.
It must therefore be noted that the Euro 5 and especially Euro 6 emissions levels applicable to heavy commercial vehicle engines can only be achieved with considerable additional technical and economic expenditure. Due to catalytic converter-related increased exhaust gas back pressure and the adaptations of the combustion process that may be required, a degradation of the fuel consumption performance can be expected for all combinations of methods for reducing NOx and particle emissions. What is being sought here in principle, however, are NOx particle emissions from the engine that are as low as possible, since that reduces the effort required on the part of exhaust gas treatment.
Besides measures taken on the interior of the engine and the use of exhaust gas treatment systems, composition-modified fuels also represent an attractive possibility, in principle, for reducing harmful emissions in diesel engines. Special interest has long existed in the addition of water and other components, such as alcohol, to diesel fuel, since the nitrogen oxide-soot trade-off can be favorably influenced in this way, cf. Bach, F., Luft, M., Bartosch, S., Spicher, U.: Einfluss von Diesel-Ethanol-Wasser-Emulsionskraftstoffen auf die Dieselmotor-Emmissionen [Influence of diesel-ethanol-water emulsion fuels on diesel engine emissions]. MTZ 05/2011, pp. 408-414.
When using water-in-diesel fuel emulsion, either a ready-to-use water-diesel emulsion is injected into the combustion chamber instead of pure diesel fuel while using an emulsifying additive or an emulsion produced on board the vehicle by the existing injection system. The production of the emulsion in the vehicle has the advantage that the water fraction in the mixed fuel can be selected relatively freely in consideration of the technical limits of combustion with regard to the maximum reduction of pollutants.
In principle, besides the use of water-in-diesel fuel emulsions, it is also possible to exploit the advantageous characteristics of water to reduce combustion temperatures through the injection of water into the intake air and the direct injection of the water into the combustion chamber. Due to the high enthalpy of vaporization of the water, when it is added through the intake pipe and especially when it is injected directly, substantial cooling of the intake air or combustion air is achieved in the cylinder and thus also a reduction in nitrogen oxide emissions of up to 50 percent. Due to the relatively little mixing of the diesel fuel with water in the combustion chamber and the resulting lesser homogenization of the diesel fuel in the combustion chamber in comparison to emulsion fuels, however, the reduction of the soot emissions ends up being less; cf. DE 10 2009 048 223 A1. Emulsion fuels therefore offer, in addition to easier use in series engines, greater potential for reducing the critical harmful components in the exhaust gas of diesel engines.
Water-diesel emulsions can be regarded as disperse multi-phase systems of at least two liquids insoluble in a mixture in which water is regarded as the inner, disperse phase. Accordingly, diesel fuel represents the outer phase, the dispersant. Water-diesel emulsions are not thermodynamically stable and separate after a relatively short standing time. Through the use of emulsifying additives, so-called emulsifiers, it is possible, in principle, to convert a water-diesel emulsion into a thermodynamically stable form. One criterion that is important for the suitability of an emulsion as fuel for diesel engines is that the water droplets be distributed as finely as possible in the diesel fuel. Emulsions produced in the vehicle using an emulsifier or, if no emulsifier is used, using an appropriate mixing device are suitable for mobile use. Emulsions produced outside of the vehicle, such as those available at gas stations, for example, have a constant composition that is not adapted to the requirements of the engine operation and therefore does not achieve full potential in terms of the reduction of emissions and consumption.
The effect of water-diesel emulsions consists, on the one hand, in a temperature reduction occurring during the water vaporization and, on the other hand, in reduced combustion temperatures as a result of the increased inert gas fraction in the form of water vapor. Both lead to a lengthening of the physical ignition delay, which leads to a more uniform (homogeneous) distribution of the fuel in the combustion chamber and thus to a greater proportion of “pre-mixed” combustion. The resulting intensified homogenization of the mixture in conjunction with the water droplets being finely distributed in the emulsion leads to a reduction of highly fuel-rich regions of the mixture, which are substantially responsible for the occurrence of soot during the combustion process. The reduction of the nitrogen oxide emissions can be attributed to a significant flame temperature reduction both as a result of the high enthalpy of vaporization of the water and the water-related lower local specific heat release in the combustion zone; cf. Pittermann, R., Hinz, M., Kauert, L: Einfluss von Abgasrückführung und Kraftstoff-Wasser-Emulsion auf Verbrennungsablauf und Schadstoffbildung im Dieselmotor [Influence of exhaust gas recirculation and fuel-water emulsion on combustion process and pollutant formation in the diesel engine]. MTZ 60(1999)12, pp. 812-818. The frequently used exhaust gas recirculation (EGR) for reducing the NOx emissions also brings about lower flame temperatures in accordance with the increased inert gas fraction. However, increased soot emissions occur at higher EGR rates, which can be avoided in the combination with water-diesel emulsion fuels. The use of water-diesel emulsion fuels thus increases EGR tolerance and hence the potential for reducing NOx and soot.
Another requirement for the optimal use of an emulsion fuel is the need to adapt the water fraction in the emulsion to different engine operating states, and to engine shutdown and startup even after extended non-operation.
In the starting phase of the diesel engine, reliable and quick startup and quick heating of the engine can only be achieved in pure diesel fuel operation, since stable combustion is already achieved after several stroke cycles. When using a water-diesel emulsion in the starting phase as well, the number of stroke cycles without combustion increases as a result of the worsened self-ignitability of the emulsion, with commensurately increased emissions of uncombusted fuel. As the engine warms up, the water fraction in the emulsion can be increased in the warm-up phase.
In the case of predominantly stationary engine operation and high output, a greater proportion of water can be contained in the emulsion due to the higher combustion chamber temperatures in order to thus achieve a combustion process that is as efficient as possible along with simultaneously high reduction of NOx and particle emissions. In low load states and commensurately low combustion chamber temperatures, a reduction of the water fraction in the emulsion is necessary in order to prevent excessive cooling of the flame zones and the associated emissions of uncombusted fuel. Predominantly stationary engine operation with only relatively slow changes in load and engine speed does not require dynamic emulsifying systems.
In principle, the use of the full potential of a water-diesel emulsion in terms of reduced NOx and soot is only possible if the water fraction is as close as possible to the respective technical combustion limit, as a function of the operating point. For the dynamic operation that usually occurs with automobiles, this means that it is absolutely necessary to have a very quick adaptation of the water fraction to the momentary combustion chamber temperatures and to the oxygen content available for combustion while making use of exhaust gas recirculation. The quicker the adaptation of the water fraction to the momentary operating state of the engine, the greater the reduction in emissions. This is all the more important given that the determination of the emissions behavior of diesel engines is done for commercial vehicles and mobile working machines using transient exhaust gas test cycles.